The evolution of optical networks has moved toward maximum time-flexibility, such that transceivers are able to maximize spectral efficiency (SE) by adapting to the actual conditions of the network and data rate for the current traffic demand. To simplify transceiver implementation, the channel spectral allocation Δf and symbol rate Rs may be kept as constants. The use of a standard modulation format with fixed bit-per-symbol (BpS) results in fixed increments of SE, since SE=BpS·Δf/Rs.
The reach of a coherent optical transmission system may be further limited by nonlinearity in the optical fiber path generated by both the optical channel itself, and by other channels at different optical wavelengths. In a polarization multiplexed transmission, the nonlinearities may also be due to intensity fluctuations in the orthogonal polarization.